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Diffusion and Osmosis
- Membrane functions
- Membrane structure
- Diffusion
- Osmosis
- Tonicity
- Junctions between cells
L. 2. 10.09.10
MEMBRANE FUNCTIONS
•Barrier and protection from the environment
•Compartmentalization
subcellular compartments
(nucleus, mitochondria, endoplasmic reticulum, etc.)
•Selective permeability
diffusion (passive and active)
active transport (expend energy)
•Regulate movement of substances in/out of cells
regulation of Intracellular Fluid [ ICF ]
regulation of Extracellular Fluid [ ECF ]
regulation of chemical composition ICF  ECF
•Recognition, Communication
MEMBRANE FUNCTIONS
Fig. 2.2 Storage of potential energy in electrochemical gradient.
Composition of ICF and ECF
Water
~75%
by weight
~99%
of total molecules
Inorganics ~0.75%
of total molecules
Organics
~0.25%
of total molecules
In the body, these two compartments are always in osmotic
equilibrium, even though the composition of the fluids in them is very
different.
Ionic concentrations
in vertebrate skeletal
muscle (mmoles)
Intracellular Fluid
Location: The distinction between ICF and ECF is clear
and is easy to understand: they are separated by the cell
membranes.
Composition: Intracellular fluids are high in potassium
and magnesium and low in sodium and chloride ions.
Behaviour: Intracellular fluids behave similarly to
tonicity changes in the ECF.
http://www.anaesthesiamcq.com/FluidBook/fl2_1.php
Extracellular Fluid
The ECF compositional similarity is in some ways, the opposite of that for the ICF
(low in potassium & magnesium and high in sodium and chloride). The ECF is
divided into several smaller compartments. These compartments are distinguished
by different locations and different kinetic characteristics:
Interstitial fluid (ISF) consists of all the bits of fluid which lie in the
interstices of all body tissues.
Plasma is the only major fluid compartment that exists as a real fluid
collection all in one location. It differs from ISF in its much higher protein
content and its high bulk flow (transport function).
The fluid of bone & dense connective tissue is significant because it
contains about 15% of the total body water.
Transcellular fluid is a small compartment that represents all those body
fluids which are formed from the transport activities of cells. It is contained
within epithelial lined spaces. It includes CSF, GIT fluids, bladder urine,
aqueous humour and joint fluid.
http://www.anaesthesiamcq.com/FluidBook/fl2_1.php
Body Fluid Compartments (70 kg male)
ECF
Plasma
ISF
Dense CT
water
Bone water
Transcellular
ICF
TBW
% of Body
Weight
27
4.5
12.0
% of Total Volume
Body Water (Litres)
45
19
7.5
3.2
20.0
8.4
4.5
7.5
3.2
4.5
1.5
33
60%
7.5
2.5
55
100%
3.2
1.0
23
42 liters
http://www.anaesthesiamcq.com/FluidBook/fl2_1.php
Biological membranes
phospholipid bilayer
cholesterol
proteins
Fig.2.43
Phosphoglyceride (phospholipid)
Hydrophilic
Lipophobic
Water soluble
Hydrophobic
Lipophilic
Water insoluble
Fig.2.36
Permeability of membranes to polar and nonpolar molecules…
Phosphoglycerides:
- Phosphatidylcholine
- Phosphatidylserine
- Phosphatidylethanolamine
Membranes also possess other lipids, including
sphingolipids, glycolipids.
Dissolving salts (NaCl) in water
The Fluid Mosaic Model of Membranes
•fluid structure is maintained by hydrophobic forces
•flexible, with lipid molecules moving freely within membrane
•cholesterol stabilizes membrane
restrains phospholipid movement
prevents close packing
membrane is less fluid but mechanically stronger
•lipid bilayer impermeable to ions and most polar molecules
•transmembrane protein-lined channels
•selective permeability due to specificity of protein channels
Membrane fluidity
– Environmental conditions affect membrane fluidity
• For example, low temperature increases van der Waals
forces between lipids and restricts movement
– Homeoviscous adaptation
• Cell keeps membrane fluidity constant by altering the
lipid profile
Functions of Membrane Proteins
•ion channels, pumps, receptors,
•recognition
•conduct bioelectric impulses
•release neurotransmitters
•respond to secretory products
•electron transport
•proteins also move laterally
protein composition differs between inner/outer side
2.47
Composition of membranes varies
•among organisms
•among tissues within an organism
•between inner and outer membrane leaflet
% protein
% lipid
Human
RBC
Human
myelin
40
43
18
79
…endocrine cells, immune cells…
…function/structure...
Membrane transport
•Passive diffusion
•Facilitated diffusion
•Active transport
Fig. 2.48
Distinguished by:
direction of transport
nature of the carriers
role of energy in the process
Fig. 2.49 Carriers involved in facilitated diffusion
Voltage-gated
channels are opened
or closed in response
to membrane potential
(K+ channels open
when the membrane
depolarizes)
Ligand-gated channels
are opened when
specific regulatory
molecules are present
(Ca2+ channel that is
sensitive to inositol
triphosphate (IP3)).
Mechanogated
channels are regulated
through interactions
with the subcellular
proteins that make up
the cytoskeleton.
DIFFUSION
Diffusion - the process by which molecules
spread from areas of high concentratiion, to
areas of low concentration. When the
molecules are even throughout a space - it is
called EQUILIBRIUM
DIFFUSION
Fundamental process in movement of substances in
biological systems
Diffusion processes of physiological importance
occur over very short distances
e.g. diffusion of nutrients
intestinal lumen → intestinal epithelium →
intestinal capillaries
e.g. diffusion of CO2 and O2 at respiratory
epithelia
Diffusion time α d2
if O2 diffuses 0.1 mm in 1 sec
1 mm in 100 sec
Rate of diffusion:
dQ = DA [dC/dX]
dt
rate
diffusion . area
coefficient
FICK DIFFUSION EQUATION
p.29
. concentration
gradient
In biological systems, simplified to:
dQ
dt
=
P
moles/cm2/s
(CI - CII)
For simple diffusion of non-electrolyte
(linear function)
permeability
constant
cm/s
PERMEABILITY
.
concentration
difference
moles/cm3
 diffusion coefficient (membrane,
solute)
 partition coefficient
1/ membrane thickness
Movement of charged particles across membranes
•membrane permeability to the particle
•electric potential across membrane
•chemical gradient across membrane
J – rate of diffusion
or flux (M.cm-2.s-1)
Donnan equilibrium :
=
[K+]I [Cl-]II
[K+]II [Cl-]I
Osmosis - the
diffusion of water
(across a
membrane)
OSMOSIS
Fig. 2.8
In biological systems:
solvent is water
solute permeability depends on
1) membrane properties
2) solute properties
Water flow across a semipermeable membrane generates
hydrostatic pressure
OSMOLARITY
- the ability of solutions to induce water to cross a membrane.
The force associated with the movement of water is the osmotic
pressure (mOsm in biological systems, osmoles per liter)
FOUR BASIC COLLIGATIVE PROPERTIES OF SOLUTES (p.29)
depends on number of dissolved particles NOT their
chemical identity
·
osmotic pressure
·
freezing point (FP)
·
boiling point (BP)
·
vapour pressure (VP)
1.00 mole in 1000 g H2O =1.00 molal (1m)
1.00 mole in 1000 mL solution =1.00 molar (1M)
1.00 m solution of a non-electrolyte:
depresses FP by -1.86oC
elevates BP by 0.54oC
has VP of 22.4 atm
For non-electrolyte (glucose):
osmolarity = molarity
For electrolyte:
osmolarity > molarity
strong electrolytes almost fully dissociate,
especially in weak solutions typical of
biological systems (e.g. NaCl, KCl - 2 OsM)
molar concentration (also called molarity, amount
concentration or substance concentration) is a measure of
the concentration of a solute in a solution, or of any
molecular, ionic, or atomic species in a given volume.
TONICITY
- the effect of a solution on cell volume
•response of cell when immersed in solution
•animal cells not surrounded by rigid cell walls
•shrink or swell in response to osmotic flow
Net H2O
movement
Cell volume
None
Unchanged
In
Swells
Out
Shrinks
OSMOLARITY versus TONICITY
Fig. 2.9
How is cell volume regulated?
…why the differences…
Solution requires preventing the accumulation of Na+ in cell
PASSIVE DIFFUSION
– Lipid-soluble molecules
– No specific transporters are needed
• Molecules cross lipid bilayer
– No energy is needed
– Depends on concentration gradient
• High concentration  low concentration
• Steeper gradient results in faster rates
PASSIVE DIFFUSION
•crossing aqueous-lipid-aqueous barriers
•importance of lipophilicity of the substance
K – partition coefficient
(Kow)
•importance of hydrogen bonding and -OH
PASSIVE TRANSPORT (Facilitated diffusion)
•Diffusion in aqueous phase through membrane channels
<1.0 nm diameter e.g. aquaporins
•Carrier-mediated passive transport
facilitates movement of polar hydrophilic substances
(e.g. glucose, amino acids)
specificity
no ATP expenditure
Types of carrier proteins
selective
e.g. Cystic fibrosis –
defective chloride transport channel protein
Importance of diffusion in
biological systems
…examples…
•Nutrients
•Respiratory gases
•Metabolic wastes
Active transport
• Two main types of active transport
– Primary active transport
• Direct use of an exergonic reaction
– Secondary active transport
• Couples the movement of one molecule to the
movement of a second molecule
– Distinguished by the source of energy
– Primary active transport
•movement AGAINST concentration gradient
•requires energy from ATP
•requires protein carrier
acts as an ATPase
selective
1.
2.
3.
4.
5.
6.
X bonds to binding site on carrier
Bonding hydrolyzes ATP to ADP + Pi
Phosphorylation of carrier
Conformational change in carrier
X exposed to other side of membrane
X detaches
– Hydrolysis of ATP provides energy
• Transporters are ATPases
– Three types
• P-type
– Pump specific ions (e.g., Na+, K+, Ca2+)
• F-type and V-type
– Pump H+
– ABC type
– Carry large organic molecules (e.g., toxins)
COUPLED TRANSPORT
(cotransport; secondary active transport)
"uphill" movement of solute A driven by "downhill" diffusion
of another solute B, therefore using energy stored as ion
gradients.
Symporter: A & B cross membrane in
same direction.
Antiport or exchanger
carrier: molecules
move in opposite
directions
Maintenance of differential transmembrane solute
concentrations (disequilibrium between ECF and ICF) in
all living cells require continual expenditure of energy to
counteract equalizing effect of diffusion
SODIUM – POTASSIUM PUMP
(high Na+ in ECF, high K+ in ICF)
Fig. 2.2 Storage of potential energy in electrochemical gradient.
Transport of Macromolecules
Endocytosis
- pinocytosis
(ingestion of fluids)
-phagocytosis
(ingestion of solids)
Exocytosis
release of material
Fig. 2.54
JUNCTIONS BETWEEN CELLS
GAP JUNCTIONS
cells coupled metabolically and
electrically via hydrophilic channels
Passage of:
inorganic ions
small water-soluble molecules:
amino acids
sugars
nucleotides
electrical signals
-labile: close in response to
high [Ca2+]ICF or high [H+]ICF
TIGHT JUNCTIONS
Cells sealed together to occlude ECF
Extracellular Matrix
 Gel-like “cement” between cells
 Cell membranes are bonded to the matrix
 Insect exoskeleton, vertebrate skeleton, and mollusc
shells are modified extracellular matrices
 Molecules of the matrix are synthesized within the
cells and secreted by exocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Extracellular Matrix
Molecules of the extracellular matrix




Proteins
Glycoproteins
Glycosaminoglycans
Proteoglycans
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Extracellular Matrix
Cells can break down the extracellular matrix with
matrix metalloproteinases
Cells can move through tissues by controlling the
production and breakdown of the matrix
 For example, blood vessel growth and penetration
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings